In industry, it is often necessary to remove magnetic particles from liquids such as e.g. motor oils, cooling liquids, water/drinking water, fuels, pumping liquids, hydraulic liquids, electrolytic wastewater, and the like. For example, US 2010012567A1, CN2758494Y, and US2005205 481 A1 disclose the use of magnets in compact form for separation of iron particles from motor oils. In addition, magnet arrangements are often used in order to remove rust particles, as disclosed e.g. in US2010065504A1.
Many of the industrial filter devices that are frequently of a multi-step configuration also have in addition a magnetic or electromagnetic separator in order to remove magnetic particles from the exhaust gas, as disclosed e.g. in US2005241484A1.
Removal of metallic or ferritic particles from wastewater of various origin has also been reported several times, for example, in WO2008101352A1.
CN201362630Y discloses a method for removal of heavy metals from wastewater wherein magnetic effects are being used also.
In further publications, the typical magnetic arrangements in the form of compact rods or plates have been abandoned. For example, in CN101733965A ferrites that are enveloped by silicone as “core-shell” systems are disclosed, and active carbon loaded with ferrites is known from US2010155335A1.
In the environmental field/water protection, separation methods that are based on magnetic effects are also used already, For example, CN101708881A discloses a method for removing blue green algae from bodies of waters in which first a magnetic flocculent is added which adheres to the blue green algae. In this way, it is possible to remove the “magnetized” blue green algae from the water by means of magnets.
It is known to produce ultra-thin fibers (so-called nanofibers, i.e., fibers with a fiber diameter of mostly (99.9%) in a range of <500 nm) by the so-called electrospinning method. The electrospinning method (also referred to as electrostatic spinning) is a versatile method in order to produce from solutions and melts, primarily of polymers, continuous fibers with diameters of a few millimeters to a few nanometers. The method is versatile regarding uses because almost all soluble and meltable polymers can be used, the polymers moreover can be furnished also with different additives, from simple soot particles to complex species such as enzymes, viruses, and bacteria, and, of course, chemical modifications are possible also.
In the proper meaning, the electrospinning process is not a fiber spinning method but instead is a method of coating with microfiber or nanofiber nonwovens. By electrospinning, it is thus possible to coat almost any solid and liquid substrates with a thin layer of polymer fiber nonwoven that typically have a weight per surface area of <1 g/cm2. Such electrospun polymer fiber nonwovens are of a pronounced delicate structure and can essentially be used only on substrates. By a significant increase of the productivity of the electrospinning method, self-supporting electrospun fabrics are now accessible also that have significantly higher weights per surface area (up to 200 g/cm2) and thus can no longer be viewed as coatings. The nanofiber filters that are produced in this way exhibit a high filtration efficiency.
In his dissertation (Herstellung wasserfester funktionaler Nanofasern durch Elektrospinnen wässriger Formulierungen [translation: Manufacture of water-resistant functional nanofibers by electrospinning of aqueous formulations], Philipps Universität Marburg, 2009), Röcker discloses the manufacture of nanofibers with a ferritic magnet particles. However, the manufacture of nanofibers that have magnetic particles as additives is very complex. On the one hand, for this purpose only very small ferrite particles can be employed; on the other hand, these particles do not uniformly distribute in the nanofibers. Moreover, in case of large-scale technical production there is the problem of deposition of ferrite particles on iron parts, for example, the wire electrodes for spraying the nanofibers; this prevents an effective coating action.
Therefore, tests have been carried out (Max von Bistram; Strukturierte funktionelle Nanofasern durch Elektrospinnen [translation: Structured functional nanofibers by electrospinning], dissertation, Philipps-Universität Marburg, 2007) to produce magnetic particles in situ. For this purpose, polyacrylonitrile (PAN) solutions, for example, were spun that contain iron-III- and optionally iron-II-ions, for example, in the form of iron-III-nitrate, Mohr's salt or complex compounds such as iron-III-acetylacetonates. The disadvantage of such an approach is that only relatively poisonous solvents can be employed for the spinning solution. Also, the iron-III-compounds must be subsequently decomposed thermally to the appropriate magnetic compounds such as Fe3O4 or γ-Fe2O3. As disclosed in Bistram, in doing so, the PAN fibers are also decomposed to pure carbon fibers, i.e., carbonized. However, carbonization produces poisonous hydrocyanic acid. Also, in case of introduction of ferrites into fine fibers/nanofibers there is the risk of washing out the ferrites because the diameters of the ferrite particles is often larger than the diameters of the fibers.
Therefore, there is a need for a magnetic filter medium or a method for its manufacture with which the aforementioned disadvantages of the prior art can be overcome.